It has been shown that certain conjugated polymers show a relatively high quantum efficiency for the radiative decay of singlet excitons. Of these, poly-p-phenylene vinylene (PPV) can be prepared via a solution-processible precursor polymer, and although itself intractable and not easily processed, can be prepared in the form of thin films of high quality by thermal conversion of the as-prepared films of the precursor polymer. Details of this general synthesis method are given in "Precursor route poly(p-phenylene vinylene): polymer characterisation and control of electronic properties" D. D. C. Bradley, J. Phys. D: Applied Phys. 20, 1389 (1987), and "Spectroscopic and cyclic voltammetric studies of poly(p-phenylene vinylene) prepared from two different sulphonium salt precursor polymers" J. D. Stenger-Smith, R. W. Lenz and G. Wegner, Polymer 30, 1048 (1989). Measurements of photoluminescence, PL, have been reported by for example "Optical Investigations of Conjugated Polymers" R. H. Friend, J. Molecular Electronics, 4, 37 (1988), and "Photoexcitation in Conjugated Polymers" R. H. Friend, D. D. C. Bradley and P. D. Townsend, J. Phys. D 20, 1367 (1987). In our earlier International Patent Application No. PCT/GB90/00584 films of PPV are disclosed as being useful as the emissive layer in a structure exhibiting electroluminescence (EL). This structure requires injection of electrons and holes from either side of the active (i.e. emissive) region of the film, and various metallic contact layers can be used. In sandwich-like structures, and for emission from the plane of the device, one of these should be semi-transparent.
The advantages of using polymers of this type as the emissive layer in EL structures include:
(a) ease of fabrication of large area structures. Various methods are available for solution-processing of the precursor polymer, including spin-coating from solution which is the preferred method, and dip-coating; PA1 (b) intractability of the polymer film, giving desirable strength, resistance to degradation from heat and exposure to oxygen, resistance to structural changes such as recrystallisation and shrinkage, and resistance to ion migration; PA1 (c) intrinsically good properties for luminescence, including low densities of charges and/or spin-carrying defects. PA1 RO--, PA1 RS--, PA1 ArO--, PA1 ArS--, PA1 NC--, ##STR1## RSe--, HO--.
However, a severe restriction has been placed on the use of such polymers by virtue of the fact that each continuous polymer film has the same characteristics throughout. That is, the quantum efficiency, wavelength of radiation and refractive index are the same over the whole surface of the film.
It is an object of the invention to provide a method of forming a polymer layer having different regions of different characteristics.